It has been long demanded to develop a strippable pressure-sensitive adhesive which is suitable for temporary fixing of ceramic sheets, electronic parts, etc. or for temporary surface protection of metallic plates, etc. and, after use, easily stripped off the adherent. Requirements for the strippable pressure-sensitive adhesives have been changing with the times and ever being made severer.
For example, there is a demand for a temporary fixing material which is easily re-adhered for re-positioning. The adhesive for this use is required to have sufficient adhesive strength for fixing of an adherent, such as a ceramic sheet or an electronic part, while capable of being released from the adherent without causing damage to the adherent for positional correction, and to be easily released from the adherent after use.
There is another demand for a surface protective material which tightly follows the shape of an adherent during forming, for example, drawing. For this use, the adhesive is required to hold an adequately controlled balance between adhesion and strippability while taking into consideration an increase of adhesive strength with time. Excessive adhesion makes smooth stripping after use impossible.
Known strippable pressure-sensitive adhesives include those comprising a base polymer having a small dynamic modulus of elasticity (200,000 dyne/cm.sup.2 or less) at a high temperature (150.degree. C.) having incorporated therein a blowing agent as disclosed in JP-A-56-61468 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). These adhesives, however, show insufficient reduction in adhesion for easy release. In some cases, they increase their adhesiveness when heat treated, for example, from 350 g/20 mm to 450 g/20 mm and are therefore unsatisfactory in strippability.
It has been proposed to use a polymer having a small dynamic modulus of elasticity at ambient temperature as a base polymer so as to improve the properties of reducing adhesion on heat treatment as disclosed in JP-A-61-174857. However, the adhesives exhibit so strong adhesion at ambient temperature that they cannot be stripped for re-adhesion without causing damage to an adherent, such as a ceramic sheet.
On the other hand, JP-A-63-17981 discloses an ultraviolet-curing pressure-sensitive adhesive containing a blowing agent, which undergoes curing and blowing on ultraviolet irradiation and heating. However, the range of application of this adhesive is greatly limited because an ultraviolet emitter is needed in addition to a heating apparatus, application to a large-sized adherent is difficult, and adhesiveness reduction treatment is difficult due to the problem of shadow. Further, it is necessary to adjust the balance between curing treatment and blowing treatment, making the operation complicated and inefficient.